The present invention relates to nondispersive, infrared gas analyzers.
Gas analyzers of the type to which the invention pertains include, for example, two radiation beams and branches, synchronous modulation for both of the branches, and chambers or cells in the branches, one for the measuring gas, the other one for a reference gas. The two radiation beams are separately received, each by a two-level detector being comprised of two chambers filled essentially with gas of the type to be detected and being sequentially traversed by the respective beam. A differential pressure gauge (membrane capacitor) is connected to these detector chambers in various ways to generate a signal representing the concentration of the particular component in the measuring gas. Analyzers of this type are, for example, disclosed in U.S. Pat. Nos. 3,725,702 and 4,156,812 and German Pat. No. 1,302,592.
Analyzers operating on the basis of frequency-selective absorption of radiation by the component to be detected produce measuring errors if any other gas in the system has an absorption band or line that overlaps any band or line of the component of interest. Obviously, the error will be the more pronounced, the lower the concentration of the desired component and the higher the concentration of the other gas. If that interference gas is present at exactly fixed amounts and concentrations, it can readily be eliminated during nulling of the instrument. The situation is different if the interfering component varies in concentration.
The problem, also called "cross sensitivity", has been recognized and dealt with in various ways; see, for example, U.S. Pat. No. 3,925,667 and German Pat. No. 27,02,744. The U.S. Pat. No. 3,925,667, in particular, deviates from the overall layout, as described, in that both beams traverse the measuring gas equally; and the detector portion for one beam has but one detection chamber.
It is an object of the present invention to permit retention of the overall layout, and particularly the two chamber-per-branch detector arrangement, and still reduce the cross sensitivity.